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Proceedings of the Poycd Society 
course, when once formed, is comparatively little affected by the 
chemical activity of rain-water. Hence the conservation of the even 
surface of the marble. It is liable, however, to he cracked by an 
internal expansion of the stone, to which I shall immediately refer, 
and also to rise in small blisters, and, as I have said, its rupture 
leads at once to the rapid disintregation of the stone. 
The cause of this disintegration is the next point for consideration. 
Chemical examination revealed the presence of a slight amount of 
sulphate in the heart of the crumbling marble; hut the quantity 
appeared to me to he too small seriously to affect the cohesion of 
the stone. I submitted to microscopic examination a portion of a 
crumbling urn of white marble in Canongate Churchyard. The 
tomb hears a perfectly fresh date of 1792 cut in sandstone over the 
top ; hut the marble portions are crumbling into sand, though the 
structure faces the east, and is protected from vertical rain by arch- 
ing mason-work. A small portion of the marble retaining its crust 
was boiled in Canada balsam, and was then sliced at a right angle 
to its original polished surface. By this means a section of the 
crumbled marble was obtained, which could he compared with one 
of the perfectly fresh stone (see fig. B). From the dark outer amor- 
phous crust, with its carbonaceous and other miscellaneous particles, 
fine rifts could he seen passing down between the separated calcite 
granules, which in many cases were quite isolated. The black crust 
descends into these rifts, and likewise passes along the cleavage planes 
of the granules. Towards the outer surface of the stone, immediately 
beneath the crust, the fissures are chiefly filled with a yellowish 
structureless substance, which gave a feeble glimmering reaction 
with polarised light, and enclosed minute amorphous aggregates like 
portions of the crust. It probably consists chiefly of sulphate of 
lime. But the most remarkable feature in the slide was the way in 
which the calcite granules had been corroded. Seen with reflected 
light they resembled those surfaces of spar which have been placed 
in weak hydrochloric acid to lay hare enclosed crystals of zeolites, 
The solution had taken place partly along the outer surfaces, so as 
to produce the fine passages or rifts, and partly along the cleavage. 
Deep cavities, defined by intersecting cleavage planes, appeared to 
descend into the heart of some of the granules. In no case did I 
observe any white pellicle such as might indicate a re-deposit of 
